Overcurrent releases are used in low-voltage circuit breakers to detect an overcurrent event (for example a short circuit) in good time when such an event occurs, and/or to prevent damage or destruction of the low-voltage circuit breaker by a suitable device/method, for example a device/method for immediate disconnection of the contacts.
The current flowing through the low-voltage circuit breaker is determined for this purpose. According to the prior art, this can be achieved, for example, by way of air-cored coils which are arranged close to the current-carrying elements of the low-voltage circuit breaker. The conductor through which current flows produces a magnetic field whose rate of change leads to a voltage in the coil. This voltage signal is normally converted by means of an analog/digital converter to a digital signal, which is integrated and represents a parameter for the current flowing through the low-voltage circuit breaker within a time interval.
This digitized current signal is supplied to a microprocessor, which evaluates the individual, successive current signals. If a more than proportional current rise (an overcurrent event) is now deduced from the digitized current signal, the microprocessor produces a signal which activates a release for short-circuit protection.
One particular problem in the case of electronic overcurrent releases is their susceptibility to interference. The introduction of interference through power supply units or EMC influences can corrupt the digital current signals. The use of capacitors is known, by way of example, in order to make it possible to filter out corrupted current signals.
However, a highly complex and thus costly filter mechanism would be required in order to provide the capability to effectively filter out all interference signals. For this reason, it is known for interference signals to be filtered out not only by way of electrical circuits (for example capacitors) but also after digitization of the current signals. Filtering of interference signals is important for an assessment of whether an overcurrent event has occurred, because spurious tripping of the overcurrent release could occur if the interference signals were not filtered.
In order to filter the already digitized current signals, it is known for the value (current level) of a digitized current signal to be compared with three times the value of the previous digitized current signal. If the current level is more than three times the previous current level, this was filtered out according to the prior art, since it was assumed that the current level had been influenced by interference. Filtering includes that this (filtered) current level is no longer used for current detection and calculation, and thus for assessment of whether an overcurrent event has occurred.
The known method has the disadvantage that interference can be identified only when the current signals are not greater than one third of the measurement range of the A/D converter. Otherwise, all interference is identified as being valid and is included in the current detection and calculation, so that spurious tripping of the overcurrent circuit breaker can occur. A further disadvantage is that the only interference which can be identified is that which has a considerable influence on the current levels (three times the previous current level). Relatively minor interference which does not reach three times the previous current level can, however, likewise lead to spurious tripping of the overcurrent release, if it occurs repeatedly.